Generally, prior art high voltage, high power DC power supplies have employed transformers having a high primary to secondary turns ratio, wherein the transformer is excited by a relatively low frequency, low voltage AC source. High voltage AC developed across the transformer secondary winding is converted into DC. The AC excitation is limited to relatively low frequencies, typically less than 10 kHz, because the transformer high turns ratio causes parasitic impedances in the secondary winding circuit to be reflected to the primary winding by a factor equal to the square of the turns ratio. In consequence, the parasitic impedances in the transformer secondary circuit are reflected to the transformer primary circuit as significant impedances at relatively high frequencies, to limit the transformer bandwidth to 10 kHz or less. Hence, the excitation frequency of the transformer is so limited, resulting in components having relatively large volume, weight and cost.
The structure disclosed in my copending, commonly assigned application 07/450,183, filed Dec. 13, 1989, now U.S. Pat. No. 5,023,768 for High Voltage High Power DC Power Supply, to a large extent, obviates many of the problems associated with low frequency excitation. In my aforementioned application, a high voltage, high power DC power supply includes an elongated central primary winding, surrounded by plural AC to DC converter stages, having DC output voltages that are stacked together to derive the desired high voltage, high power output. Each stage includes a separate secondary winding inductively coupled to the primary winding at a different position along the primary winding. AC induced in the secondary winding of each stage is converted into DC by a rectifier circuit, that may include voltage doubler elements. The primary winding is excited by an AC source having a frequency in excess of 100 kHz, to provide supply components having relatively small volume, weight and cost. This prior art structure has a coaxial configuration, to assist in minimizing high voltage electric field stresses. It also has a relatively wide bandwidth and uniform grading along its length.
However, the prior art device is relatively long in length and preferably employs magnetic cores in each stage. The use of magnetic cores in each stage adds to the weight, volume and cost requirements of the prior art structure disclosed in the aforementioned application.
It is, accordingly, an object of the present invention to provide a new and improved high voltage, high power DC power supply that is relatively inexpensive, occupies relatively small volume and has relatively low weight.
Another object of the invention is to provide a new and improved high voltage, high power DC power supply excited by a relatively low voltage AC source having a frequency in excess of approximately 100 kHz, to enable components employed in the supply to have relatively low weight, volume and cost.
A further object of the present invention is to provide a new and improved relatively inexpensive high voltage, high power DC power supply having uniform grading and coaxial packaging, to provide high performance electrostatic stress characteristics.
A further object of the present invention is to provide a new and improved high voltage, high power DC power supply employing printed circuit windings in each of a plurality of stacked AC to DC converter stages.
Still a further object of the present invention is to provide a new and improved relatively inexpensive high voltage, high power DC power supply that is small enough to be inserted into the same housing as the housing for an envelope of a high power vacuum tube.
Yet another object of the invention is to provide a new and improved relatively inexpensive, relatively light weight DC power supply that can be inserted into an oil filled housing including an x-ray tube, in the volume previously occupied by a connector between an anode or cathode of the tube and a high voltage input terminal.
Yet another object of the invention is to provide a new and improved relatively inexpensive, relatively light weight DC power supply that can be inserted into an oil filled housing including an x-ray tube, in the volumes previously occupied by connectors between anode and cathode electrodes of the tube and a high voltage input terminal.
The Invention
In accordance with one aspect of the present invention a power supply for deriving a high voltage DC output in response to a source of AC having a low voltage comprises a structure forming an elongated magnetic flux pole having a longitudinal axis. A first coil surrounding the flux pole is adapted to be responsive to the source of for causing magnetic flux to flow along the flux pole longitudinal axis. Multiple AC to DC converter stages having DC output terminals are connected in stacked series relation with each other. Each of the converter stages includes a separate planar coil concentric with the flux pole and located in a plane at angles to the flux pole longitudinal axis. The first coil of each stage are at a different mutually exclusive non-overlapping longitudinal position along the flux pole axis and positioned so that the magnetic flux flowing longitudinally in the flux pole induces a voltage in the coil in a plane at right angles to the flux pole longitudinal axis.
Such an arrangement is well adapted for coaxial packaging of the magnetic circuit including the flux pole and the planar coil, to provide for optimum electrostatic stress and field conditions. In addition, such a configuration inherently has a wide bandwidth, capable of a flat response between 100 kHz and several mHz, to provide high frequency operation in response to an AC source of at least 100 kHz. Inherent with high frequency operation are components having relatively low volume, weight and cost. In addition, the aforementioned configuration has uniform grading and is adapted to provide a high output voltage over a short length. Reflected impedance problems associated with high turns ratio transformers are not present because of the stacked relationship. The planar coils enable the structure to have a short length, while providing a high voltage, high power output.
In the preferred embodiment, each of the planar coils is a printed circuit coil, and each of the converter stages preferably includes a pair of printed circuit coils on opposite sides of a common printed circuit board for the particular stage. The pair of coils of each stage surround the flux pole to enable coaxial packaging to be attained. The use of printed circuit coils is particularly advantageous with regard to a structure having a relatively short length and low cost.
The coils of a single stage are series connected with each other and oppositely wound on opposite sides of the printed circuit board associated with the stage. Such a configuration is highly advantageous because it obviates the need to provide connections between interior and exterior terminals of the coils. Connections to interior terminals of a printed circuit coil require a printed circuit lead to extend across the printed circuit coil, with possible deleterious results, even if the lead is on the back face of the printed circuit board carrying the coil. The possible deleterious results are more likely to occur if the coil is excited by a high frequency source. In addition, by oppositely winding the coils on opposite sides of the printed circuit board, the total AC voltage derived from the coil arrangement is twice the voltage obtained from a single coil, since the coils are oppositely wound in series aiding relationship.
To assist in minimizing size, the printed circuit board of each stage includes rectifier and filter means connected to the output terminals of the series connected coils of the particular stage. Preferably the rectifier and filter means of each stage are connected to form a voltage multiplier, to increase the DC output voltage associated with each stage.
The coils of each stage are preferably connected to each other by a plated through hole on the board associated with the stage. The plated through hole connects the interior terminals of the two windings of each stage together.
According to a further feature, components of the rectifier and filter means of each stage are positioned toward one side of the printed circuit board associated with the respective stage. The one sides of the printed circuit boards of adjacent stages carrying the components are differently directed so that the components of the rectifier and filter means of adjacent ones of the stages extend in non-interfering and non-overlapping relation with each other. The components of adjacent ones of the stages extend beyond portions of the printed circuits boards where the printed circuit coils are located. A quaternary of the printed circuit boards form a set of stacked AC-DC converters. The extending side of each member of each set is at right angles to the other extending sides of each member of that particular set. Thereby, the space for components on each printed circuit board is three times the height required for a particular stage.
Electric insulators having relatively high dielectric strength are sandwiched between the portions of the adjacent printed circuit boards where the printed circuit coils are located so that opposite faces of the insulators substantially abut against otherwise facing surfaces of portions of the adjacent printed circuit boards where the printed circuit coils are located.
Typically, the AC source is included as part of the power supply and supplies a frequency of at least 100 kHz to the first coil. The first coil is preferably a helical coil, to provide efficient coupling of the 100 kHz source to the flux pole. While a helical coil requires greater length along the flux pole than a planar coil, it does not suffer from skin effect and possible current crowding problems, which have a tendency to reduce efficiency of the relatively high current that the AC excitation source causes to flow in the first coil.
To assist in preventing current crowding in the planar coils, a low reluctance structure is provided for substantially preventing magnetic flux flowing out of the magnetic flux pole from traversing the planar coils. According to one embodiment, the magnetic flux is substantially prevented from flowing in the planar coils by providing first and second low magnetic reluctance elements that extend radially from the flux pole to a position outside of the planar coils. The first coil and planar coils are positioned between the low reluctance elements. To minimize costs, the first and second low magnetic reluctance elements are the only elements that substantially prevent the magnetic flux from traversing the planar coils. According to a second embodiment, the magnetic flux is also prevented from flowing through the planar coils by providing a low reluctance structure that extends axially of the flux pole longitudinal axis. This structure is more expensive than a structure only employing the first and second low magnetic reluctance elements, but is more effective and prevents induction heating of components close to the supply.
To minimize the possibility of breakdown between the flux pole structure and the coils, a solid dielectric is provided between the flux pole structure and the inner surfaces of the planar coils. The dielectric has a relatively high dielectric strength to prevent the breakdown.
In one application, the invention is used in combination with an electron tube having an envelope, an anode and a cathode located in the envelope. A terminal on a housing surrounding the envelope is adapted to be connected to the AC source. The power supply, including the flux pole structure, is located in the housing between the envelope and the housing so the flux pole extends between the envelope and the housing. The planar coil deriving the lowest voltage is located closer to the housing than any of the other planar coils, while the planar coil associated with the highest voltage stage is closer to the envelope than any of the other planar coils.
In one preferred embodiment, the electron tube is an x-ray mammogram tube having anode requirements of at least 40 kV and 2-10 kW. The power supply occupies a volume in the housing where a high voltage connector of a prior art x-ray tube assembly was located.
The power supply can be located in the same housing as a grounded cathode or grounded anode x-ray fluoroscopy tube or two such supplies can be located in the same housing as an x-ray tube having high voltage anode and cathode requirements, in which case one of the supplies energizes the cathode with a high negative voltage and the other supply energizes the anode with a high positive voltage.
It is, accordingly, a further object of the present invention to provide a new and improved x-ray tube housing wherein a high voltage power supply for an electrode of the x-ray tube is located in a housing for the x-ray tube envelope.
The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of a specific embodiment thereof, especially when taken in conjunction with tee accompanying drawings.